Regulation of cytoskeletal dynamics at the immune synapse: new stars join the actin troupe

Abstract

Reorganization of actin cytoskeletal dynamics plays a critical role in controlling T-lymphocyte activation and effector functions. Interaction of T-cell receptors (TCR) with appropriate major histocompatibility complex-peptide complexes on antigen-presenting cells results in the activation of signaling cascades, leading to the accumulation of F-actin at the cell-cell contact site. This event is required for the formation and stabilization of the immune synapse (IS), a cellular structure essential for the modulation of T-cell responses. Analysis of actin cytoskeletal dynamics following engagement of the TCR has largely focused on the Arp2/3 regulator, WASp, because of its early identification and its association with human disease. However, recent studies have shown equally important roles for several additional actin regulatory proteins. In this review, we turn the spotlight on the expanding cast of actin regulatory proteins, which co-ordinate actin dynamics at the IS.

Figure 1. WASp/WIP complex function at the IS

A) Domain structure and activation of WASp. In unstimulated cells, WASp assumes an autoinhibited conformation, in which the C-terminal VCA region interacts with the GBD. Binding of Cdc42-GTP, often in conjunction with phosphorylation of WASp tyrosine 291, induces a conformational change, allowing WASp to activate Arp2/3 complex function. B) Domain structure of WIP. C) Model of WASp/WIP function at the IS. WASp and WIP are present in an obligate heterodimer generated by interaction of their N- and C-termini, respectively. WASp recruitment is mediated by the adapters SLP-76 and Nck, bringing it into proximity with Cdc42-GTP and Src kinases in the TCR signaling complex. Independent interaction of WIP with the TCR signaling complex (not shown) occurs through binding to ZAP-70 and CrkL (31). WIP and WASp likely function coordinately, with WASp activating Arp2/3-complex-induced actin poly-merization and WIP stabilizing actin filaments and perhaps also facilitating polymerization.

Figure 2. WAVE complex function at the IS

A) Domain architecture of the WAVE family of proteins. Similar to WASp, the WAVE proteins stimulate de novo actin polymerization through their VCA domains. A recent report suggests that WAVE2 actin polymerization activity is positively regulated through phosphorylation of tyrosine 150 by the Abl tyrosine kinase (47). B) Model of WAVE protein recruitment to the IS following TCR engagement. On TCR engagement, numerous intracellular signaling cascades are activated, among which are Rho family GEFs that lead to the GTP-loading of Rac1 and subsequent localization of the WAVE macromolecular complex. While the most obvious GEFs to carryout this function in T cells would be Vav1, neither Vav1 nor Rac1 has been formally shown to participate in the recruitment of the WAVE complex to the activated receptor in T cells. In addition, although the BR and Rac1 effector molecule IRSp53 have been shown to regulate the localization of WAVE to the plasma membrane in other cell types, their role in regulating the recruitment of the WAVE complex to the IS is yet to be determined. Last, recent evidence indicates that the WAVE2 complex regulates TCR-stimulated integrin activation and couples Ca²⁺intracellular store release to capacitive Ca²⁺ entrance through the CRAC channel. However, it is currently unknown how these events are controlled by the WAVE2 complex. GEFs, GTP exchange factors.

Figure 3. HS1 function at the IS

A) Domain structure of HS1. The N-terminal acidic region (A) has homology to the C-terminus of WASp and WAVE proteins and can interact with Arp2/3 complex in a limited fashion. However, actin regulatory activity is also dependent on the 3.5-HTH (cortactin) repeats and the neighboring CC region. The remainder of the protein has adapter-like properties. B) Model of HS1 function at the IS. HS1 binds Lck in resting T cells through its proline-rich region. On TCR engagement, HS1 is tyrosine phosphorylated at Y378/397, most likely by ZAP-70, stabilizing binding to Lck. This phosphorylation directs recruitment of HS1 to the IS through as yet unidentified binding partners. HS1 interacts with F-actin filaments and Arp 2/3 complex, stabilizing branched actin structures. In addition, HS1 stabilizes Vav1 recruitment at the IS, thereby fostering ongoing Arp2/3 complex activation by Cdc42/Rac effectors such as WASp and WAVE2. It remains to be determined whether HS1 binds WASp and/or WIP directly and exactly how the function of these proteins is co-ordinated at the IS.

Figure 4. Cofilin function at the IS

Cofilin is regulated by LIM-kinase-dependent phosphorylation at serine 3, which inactivates the protein. In T cells, ligation of the costimulatory molecules CD2 or CD28 induces cofilin dephosphorylation in a PI3-kinase-dependent manner, most likely by PP1/PP2 phosphatases or the slingshot phosphatase SSH1L. Once activated, cofilin binds to ADP-actin and severs the actin filament and/or depolymerizes actin monomers. The resulting monomers interact with profilin, which promotes the exchange of ADP for ATP. Profilin–ATP-actin complexes are then available for assembly at filament barbed ends. Under conditions where Arp2/3-complex-activating factors such as WASp and WAVE2 are active, this process stimulates the formation of new actin branches. Note that the effects of cofilin on filament elongation and branching are not distinct events but are two manifestations of the same activity.